Source : WordNet®

Maxwell's demon
     n : an imaginary creature that controls a small hole in a
         partition that divides a chamber filled with gas into two
         parts and allows fast molecules to move in one direction
         and slow molecules to move in the other direction through
         the hole; this would result in one part of the container
         becoming warmer and the other cooler, thus decreasing
         entropy and violating the second law of thermodynamics